Conducting poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films have high potential as flexible transparent conductive electrodes in various devices. In this study, two simple robust methanol-based treatment methods, namely, dipping process (DP) and solvent vapor annealing (SVA), were used to improve the electrical conductivity of PEDOT:PSS films in order to make them suitable as electrodes in organic solar cells (OSCs). Then, the characteristics of the methanol-treated PEDOT:PSS films were investigated. After the methanol treatments, the sheet resistance of the PEDOT:PSS films (ca. 100 Ω/Sq) were improved by more than 200 times, and the work function (∼5.0 eV) was nearly unchanged in both methods. Two completely different possible origins of the improved conductivity of the methanol-treated PEDOT:PSS films were addressed for DP and SVA approaches. Both methods were suitable for preparing methanol-treated PEDOT:PSS films that can be used as anodes for polymer-fullerene-based OSCs. The photovoltaic performance of the modified PEDOT:PSS-based devices was comparable to that of indium tin oxide (ITO)-based devices, thus demonstrating their practicality. The methanol-treated PEDOT:PSS films show great potential as flexible transparent conductive electrodes for ITO-free and metal-free devices.